This invention relates to heat transfer channels in heat exchange systems, mainly certain types of nuclear reactors, wherein a plurality of elongated heat emitting members are supported within a channel through which a heat receiving liquid passes and as a result undergoes boiling. Boiling in this context includes sub-cooled boiling, such as is allowed in pressurised water cooled nuclear reactors, as well as bulk boiling as occurs in both boiling water nuclear reactors and in steam generating nuclear reactors. The invention aims to improve heat transfer between the heat emitting members and the fluid.